Problem: $ C = \left[\begin{array}{rrr}1 & 1 & 2 \\ 0 & 0 & 1 \\ 1 & 2 & 1\end{array}\right]$ What is $ C^{-1}$ ?
Explanation: $ = \left[\begin{array}{rrr}2 & -3 & -1 \\ -1 & 1 & 1 \\ 0 & 1 & 0\end{array}\right]$